Some guns and weapons in Borderlands 3 have an Alt Fire Mode. This means that they have two methods for doing damage; regular bullet firing or Alt Mode. The Ale Mode can be vastly different depending on the weapon, but one of the most common Alt Fire Modes is to shoot grenades or explosives from your weapon instead of regular bullets. This mode is typically very good for special situations where you require some extra power, so it's good to know how to turn on and off the Alt Fire Mode.
The method for turning on Alt Fire Mode for your weapon in Borderlands 3 is to press the Down button on your D-Pad. This works on both Xbox One and Playstation 4. You can just the same thing again to turn it off.